In a recent TED talk entitled “Have We Reached the End of Physics?” the particle physicist Harry Cliff talks about two astonishingly fine-tuned numbers that are fundamental to the observed structure of our universe. He repeatedly calls these “dangerous numbers.” This is a very strange description, because as the transcript of the talk makes clear, the numbers have just the right values needed for creatures like us to exist. So why use the word “dangerous” to describe them? Given the facts Cliff discusses, it might be more appropriate to use the term “providential” to describe these numbers.
The first number Cliff discusses is the value of the Higgs field, which Cliff describes as a “cosmic energy field.” At 4:05 in the talk, Cliff says this about the Higgs field.
But there is something deeply mysterious about the Higgs field. Relativity and quantum mechanics tell us that it has two natural settings, a bit like a light switch. It should either be off, so that it has a zero value everywhere in space, or it should be on so it has an absolutely enormous value. In both of these scenarios, atoms could not exist, and therefore all the other interesting stuff that we see around us in the universe would not exist. In reality, the Higgs field is just slightly on, not zero but 10,000 trillion times weaker than its fully on value, a bit like a light switch that's got stuck just before the off position. And this value is crucial. If it were a tiny bit different, then there would be no physical structure in the universe.
This is an extreme case of fine-tuning. Physicists were so bothered by this case of fine-tuning that they spent innumerable hours (plus countless tax dollars) working on a very ornate theory called supersymmetry, designed mainly to explain away this particular case of fine-tuning. But the theory is really just one of those “rob Peter to pay Paul” affairs, as it requires the existence of a whole set of undiscovered particles, the existence of which would be just as big a case of fine-tuning as the fine-tuning that the theory tries to explain away. So far, as Cliff notes, the giant particle accelerator known as the Large Hadron Collider has failed to confirm the predictions of supersymmetry, leaving it in a state that one commentator has described as a deathbed state.
The second of Cliff's two “dangerous numbers” is the degree of dark energy in the universe. Dark energy is believed to be an energy within empty space that is causing the universe to accelerate. Cliff describes the following disagreement between theory and observations (which is actually the biggest such disagreement ever):
Now, if you use good old quantum mechanics to work out how strong dark energy should be, you get an absolutely astonishing result. You find that dark energy should be 10 to the power of 120 times stronger than the value we observe from astronomy. That's one with 120 zeroes after it. This is a number so mind-bogglingly huge that it's impossible to get your head around. We often use the word "astronomical" when we're talking about big numbers. Well, even that one won't do here. This number is bigger than any number in astronomy. It's a thousand trillion trillion trillion times bigger than the number of atoms in the entire universe.
You can get a better grip on this idea if you understand that quantum mechanics predicts that ordinary empty space should be teeming with mass-energy, so much so that each cubic meter of empty space should be denser than steel. In fact, the quantum mechanics prediction is that each thimble-sized unit of space should have more mass-energy than if the entire Earth were packed into it.
So why do we live in a universe so different from that type of universe? Physicists basically have no clue.
Ever since the invention of the atomic bomb we have enthroned our theoretical physicists as some kind of towering geniuses, but in this respect it seems that they are really like someone who predicts this as the score of the next Super Bowl:
Carolina Panthers: 345,564,456,786,123,523,236,234,845
Denver Broncos: 24
Describing the multiverse theory, Cliff offers this wobbly explanation (perhaps just summarizing the thoughts of others):
What if all of these 10 to the 500 different possible universes actually exist out there somewhere in some grand multiverse? Suddenly we can understand the weirdly fine-tuned values of these two dangerous numbers. In most of the multiverse, dark energy is so strong that the universe gets torn apart, or the Higgs field is so weak that no atoms can form. We live in one of the places in the multiverse where the two numbers are just right. We live in a Goldilocks universe.
This is basically an “anthropic principle” explanation, the idea that we can explain some lucky number in our universe simply on the grounds that if it didn't have such a number, we wouldn't exist. But unless some willful causal agent is introduced within such an explanation, such an explanation is untenable, because you can't naturally explain something merely by referring to something else that came eons later. Causes come before effects, not after them. If event X occurred millions of years after effect Y, we cannot explain effect Y merely by referring to event X. The universe's level of dark energy and the numerical value of the Higgs field are effects that existed billions of years before life appeared, and we can't explain such effects naturally merely by referring to something that came eons later (the appearance of life).
I may add that the multiverse does nothing to fix the bad reasoning of trying to explain an effect by referring to something that came eons later. Nor would the existence of a multiverse make it more likely that our particular universe would have the right numbers by lucky coincidences, since the probability of success on any one random trial is not increased by increasing the number of random trials.
Moreover, while Cliff uses the statement “suddenly we can understand the weirdly fine-tuned values of these two dangerous numbers,” his previous statements contradict that. For based on what he said earlier, a dark energy level of the type we have (very, very low) should not just be incredibly improbable but actually impossible. For quantum mechanics predicts (according to Cliff) that dark energy should be billions of trillions of quadrillions of quintillions of times greater than it is. So apparently according to quantum mechanics, the chance of a universe such as ours is not just very, very low, but actually zero.
Multiverses actually are not of any value in explaining the incredibly improbable, and they certainly are of no value in explaining a reality that seems to have a natural probability of zero.
See here for more on the "vacuum catastrophe" discussed by Cliff.